Bio-based electrolytes for supercapacitors: A sustainable path toward green energy storage

The increasing demand for sustainable and high-performance energy storage systems has significantly advanced supercapacitor (SC) technology. Among the critical components of an SC, the electrolyte plays a central role in determining device efficiency, stability, and energy density. This review offers a thorough and systematic evaluation of bio-based electrolytes developed for SC applications, focusing on their synthesis approaches, structural and electrochemical characterization, and key performance indicators. An extensive literature survey was conducted encompassing bio-inspired aqueous, non-aqueous, redox-active, and hybrid electrolyte systems. Special emphasis is placed on biomass-derived resources such as agricultural waste, algae, starch, lignin, natural rubber, cellulose, and other lignocellulosic materials, highlighting their transformation into functional and sustainable electrolyte materials. The electrochemical behavior of these bio-based systems is critically compared with conventional aqueous and non-aqueous electrolytes, while also addressing the practical challenges and future opportunities for their implementation in commercial devices. Additionally, the synergistic integration of biomass-derived electrolytes with polymers, redox additives, and advanced electrode materials is examined to demonstrate pathways for enhancing overall device performance. Importantly, bio-based electrolytes demonstrate that sustainability and high electrochemical performance can coexist, achieving competitive ionic conductivity, stability, and energy density comparable to conventional systems. Biopolymer hydrogels, eutectogels, and bio-derived ionic liquids emerge as particularly promising candidates due to their tunable properties, flexibility, and lower environmental impact. Overall, bio-based electrolytes represent a viable and forward-looking strategy for developing greener, high-performance SC technologies. Through this review, we aim to highlight the potential of bio-based electrolytes in paving the way for high-performance, greener, and more efficient energy storage technologies.